The present invention relates to optical waveguide devices which can be utilized as high speed waveguide type optical modulator at 20 Gb/s and above in high speed optical communications, optical switch or exchange network, optical data processing, optical image processing and various other systems.
Waveguide type optical modulators and switches are most important key elements when realizing high speed optical communication, optical exchange network, optical data processing, optical image processing and various other systems. Waveguide type optical modulators have been fabricated on some interesting substrates by various methods. However, the optical waveguide devices mostly includes a LiNnO.sub.3 substrate and a GaAs substrate. In-diffusion of titanium into LiNbO.sub.3 is a convenient and relatively simple method of fabricating low-loss strip waveguide having satisfactory electro-optical characteristics on a substrate. Important parameters of the waveguide type modulator are drive power, modulation bandwidth and insertion loss. The bandwidth and drive power are in a trade-off relationship to each other. Research in waveguide type modulators is concentrated on optimizing the trade-off relation.
The bandwidth of a waveguide type modulator depends mainly on the type, material and shape of electrodes, and dielectric constant of substrate. For broad-band applications, travelling wave electrodes have been widely used. The travelling wave electrode is regarded to be an extension of drive transmission line. This means that the travelling wave electrode should have the same impedance as that of the source and cable. In this case, the modulation speed is limited by the difference in the transmission times (or phase velocities or effective refractive indexes) for the optical waves and microwaves.
There are two different travelling wave electrode structures that can be used, i.e., (1) ASL (Asymmetric Strip Line) or ACPS (Asymmetric Coplanar Strip) type structure and (2) CPW (Coplanar Waveguide) type structure. In order to increase the bandwidth, the effective microwave refractive index n.sub.m has to be reduced (from a value of 4.2) to be close to the effective optical reflective index n.sub.O (typically 2.2 in case of LiNbO.sub.3 substrate).
Travelling wave modulator bandwidth is limited by phase velocity mismatch between microwaves and optical waves. This means that it is necessary to decrease the effective reflective index difference between microwaves and optical waves by reducing the effective microwave refractive index. One of the methods of reducing the effective microwave reflective index for increasing the bandwidth, is to use a thick electrode and buffer layer. Modulators using ASL or ACPS electrode structure have already been proposed, as shown in "33-GHz. cm Broadband Ti:LiNbO.sub.3 Mack-Zehnder Modulator", ECOC' 89, paper ThB22-5, pp. 433-436 (1989). Such a modulator reduces the effective microwave refractive index using a large thickness film electrode (of ASL or ACPS electrode structure) and a buffer layer. A problem in the ASL or ACPS electrode structure is that the bandwidth is limited to about 12 GHz by microwave resonance problem due to the chip cross section. In order to increase the bandwidth above 20 GHz, the chip dimensions (both width and thickness) are reduced to about 0.6 mm. There is no problem in setting the chip thickness to about 0.6 mm. However, the demand for setting the chip width to about 0.6 mm poses problems during handling, mounting and packaging the chip.
Another method of reducing the effective microwave refractive index is to use an air layer that is formed by using a metal shield for the conventional travelling wave electrode structure. This is shown in "New Traveling-Wave Electrode Mach-Zehnder Optical Modulator with 20 GHz Bandwidth and 4.7 V Driving Voltage at 1.52 .mu.m Wavelength", Electronics Letters, Vol. 25, No. 20, pp. 1382-1383 (1989). A problem in this structure is that a special shield of metal cover having a groove should be fabricated in accurate dimensions. This requires a particularly difficult technique, increases the steps of manufacture and decreases permissible fabrication tolerances.
If the phase velocity mismatch between the microwaves and optical waves can be alleviated by either of the methods described above, the bandwidth of modulator/switch is further restricted by the microwave attenuation of the electrode structure. Even if there is perfect phase velocity match between the microwaves and optical waves, the ultimate bandwidth of the device is narrow unless the microwave attenuation is reduced. Generally, the microwave attenuation in device result from the following causes:
a) Conductor loss (which is a function of electrode material and parameter thereof); PA1 b) Dielectric loss (which is a function of substrate properties); PA1 c) Loss due to impedance mismatch between 50.OMEGA.source and load; PA1 d) Loss due to higher order mode propagation (more for the case with a CPW electrode); and PA1 e) Connector loss.
Thus, a new structure or design is necessary for high speed modulators which has a characteristic impedance of about 50.OMEGA., substantially perfect phase velocity matching between microwaves and optical waves and low microwave attenuation. The new structure requires only extension of the general electrode fabrication process and ensures a simple fabrication process which does not require any extra special shield.
The inventors of the present invention earlier solved some of the above problems, and realized a wide-band modulator using a thick but conventional CPW electrode structure (a prior art example shown in FIGS. 5A and 5B, FIG. 5A being a plan view, and FIG. 5B showing a sectional view taken along line A-A' in FIG. 5A). This modulator is disclosed in "A wide-band Ti:NbO.sub.3 Optical Modulator with a Conventional Coplanar Waveguide Type Electrode", IEEE Photonics. Tech. Lett. Vol. 4, No. 9, pp. 1020-1022, 1992. According to this research, the effective microwave refractive index n.sub.m can be reduced from a value of 4.2 to be close to the effective refractive index n.sub.o (typically a value of 2.2 in case of LiNbO.sub.3 substrate). This can be realized with suitable design in the material and thickness of the buffer layer and material and thickness of the electrode. The inventors of the present application alleviated the microwave attenuation of the structure by reducing the microwave loss due to higher order mode propagation. This could be realized by reducing the chip thickness from 0.8 mm to 0.2 mm. Consequently, a wide-band modulator could be obtained.
It is further required to reduce the conductor loss of the CPW electrode. The reduction of conductor loss reduces attenuation of all microwaves of device to permit obtaining a high-speed (wide-band) modulator/switch.
In the specification of Japanese Patent Laidopen No. Heisei 6-300994 filed by the present applicant shown in FIGS. 6A and 6B, FIG. 6A being a plan view and FIG. 6B being a sectional view taken along line A-A' in FIG. 6B, an additional electrode structure (upper electrode structure) is used in addition to the existing electrode structure (lower and middle structure), thus reducing the conductor loss to increase the bandwidth. The ratio between the signal electrode width and the gap between the signal electrode and the ground electrode is maintained in the additional electrode structure in order to keep the characteristic impedance constant. The characteristic impedance is reduced due to the problem of the "edge effect"(i.e., the edge of the additional upper signal electrode being too close to the edge of the lower/intermediate ground electrode).